Electromagnetic Induction for IGCSE WJEC Physics | IGCSE WJEC 物理：电磁感应考点精讲

📚 Electromagnetic Induction for IGCSE WJEC Physics | IGCSE WJEC 物理：电磁感应考点精讲

Electromagnetic induction is one of the most important topics in the WJEC IGCSE Physics specification. It explains how a voltage can be generated without a battery, simply by moving a conductor in a magnetic field or changing the magnetic field around a coil. This principle underpins the functioning of generators and transformers, which are vital for modern electricity supply. Understanding the key factors, rules, and equations will help you master questions on induced EMF, AC/DC generators, and transformer calculations, which appear regularly in the exam.

电磁感应是 WJEC IGCSE 物理大纲中最重要的主题之一。它解释了如何在不使用电池的情况下，仅通过在磁场中移动导体或改变线圈周围的磁场就能产生电压。这一原理是发电机和变压器工作的基础，对现代电力供应至关重要。理解关键因素、定则和公式，将帮助你掌握关于感应电动势、交流/直流发电机和变压器计算的考题，这些内容在考试中经常出现。

1. What is Electromagnetic Induction? | 什么是电磁感应？

Electromagnetic induction is the process by which a potential difference (voltage) is generated across a conductor when it experiences a changing magnetic field. If the conductor forms part of a complete circuit, an induced current flows. This effect is also called the generator effect.

电磁感应是指导体在经历变化的磁场时，其两端产生电势差（电压）的过程。如果导体构成完整回路的一部分，就会有感应电流流动。这种效应也称为发电机效应。

A voltage is induced whenever a conductor cuts magnetic field lines. The conductor can be a straight wire moving between magnet poles, or a coil placed in a changing magnetic field (e.g., when a magnet moves in and out of a coil).

只要导体切割磁感线，就会产生电压。导体可以是在磁极间移动的直导线，也可以是处于变化磁场中的线圈（例如当磁铁移入或移出线圈时）。

2. Inducing Voltage in a Straight Wire | 在直导线中感应电压

Imagine a single straight wire placed between the poles of a permanent magnet. If the wire is moved so that it cuts across the magnetic field lines, a voltmeter connected across its ends will register a small voltage. The faster the wire moves, the larger the voltage. If the wire moves parallel to the field lines, no cutting occurs, and no voltage is induced.

想象一根直导线放在永磁体两极之间。如果移动导线使它切割磁感线，导线两端连接的电压表会显示一个小电压。导线移动越快，电压越大。如果导线平行于磁感线运动，则没有切割，不会产生电压。

This simple setup demonstrates three requirements: a magnetic field, a conductor, and relative motion between them. The conductor must cut field lines, not slide along them.

这个简单装置展示了三个必要条件：磁场、导体以及它们之间的相对运动。导体必须切割磁感线，而不是沿着磁感线滑动。

3. Factors Affecting the Induced Voltage | 影响感应电压的因素

Several factors determine the size of the induced EMF (electromotive force). Increasing any of them increases the voltage:

有几个因素决定感应电动势的大小。增加以下任何一项都会增大电压：

The strength of the magnetic field – using stronger magnets or more current in an electromagnet.

磁场强度——使用更强的磁铁或增大电磁铁中的电流。
The speed of movement – moving the wire or magnet faster cuts field lines more rapidly.

运动速度——更快地移动导线或磁铁能更迅速地切割磁感线。
The number of turns on the coil – a coil with many turns of wire produces a larger total voltage than a single wire.

线圈匝数——多匝线圈产生的总电压比单根导线大。
The area of the coil (if a coil is used) – a larger coil cutting more field lines induces a greater EMF.

线圈面积（如果使用线圈）——较大的线圈切割更多磁感线会感应出更大的电动势。

In an exam, you may be asked to describe how to increase the induced current in a demonstration. The same principles apply to generators.

考试中可能会要求你描述如何增加演示中的感应电流。同样的原理也适用于发电机。

4. Fleming’s Right-Hand Rule for Generators | 发电机用的弗莱明右手定则

Fleming’s right-hand rule is used to predict the direction of induced current when a conductor moves in a magnetic field. Hold the thumb, forefinger, and second finger of your right hand at right angles to each other:

弗莱明右手定则用于判断导体在磁场中运动时感应电流的方向。伸出右手，让拇指、食指和中指两两垂直：

First finger (index) points in the direction of the magnetic field (North to South).

食指指向磁场方向（北到南）。
Thumb points in the direction of the conductor’s motion.

拇指指向导体运动方向。
Second finger points in the direction of the induced current.

中指指向感应电流方向。

This rule is for the generator effect only. Do not confuse it with Fleming’s left-hand rule for the motor effect. In the IGCSE exam, clearly identify which hand you are using and why.

该定则仅用于发电机效应。切勿与电动机效应的弗莱明左手定则混淆。在 IGCSE 考试中，要清楚说明使用的是哪一只手及其原因。

5. Inducing Voltage in a Coil (Magnet and Coil) | 线圈中的感应电压（磁铁与线圈）

One of the classic demonstrations is moving a bar magnet into a coil connected to a sensitive centre-zero galvanometer. When the magnet moves, the pointer deflects, showing an induced current. When the magnet stops, the current falls to zero. Pulling the magnet out deflects the pointer in the opposite direction.

一个经典演示是将条形磁铁插入连接了灵敏中心零位检流计的线圈中。当磁铁移动时，指针偏转，表明有感应电流。磁铁停止运动，电流降为零。将磁铁拉出时，指针反向偏转。

The induced current produces its own magnetic field that opposes the motion of the magnet – this is often called Lenz’s law, though you do not need to name it. For example, if you push the north pole of a magnet into a coil, the end of the coil facing the magnet becomes a north pole, repelling the magnet and resisting the motion.

感应电流会产生自己的磁场，抵抗磁铁的运动——这通常称为楞次定律，尽管不必说出名称。例如，将磁铁北极推入线圈时，线圈面对磁铁的一端成为北极，排斥磁铁，阻碍其运动。

You can determine the coil’s current direction by treating the coil as an electromagnet and using the right-hand grip rule. If the coil’s end is a north pole, the current direction is anticlockwise when viewed from that end.

你可以将线圈看作电磁铁，用右手螺旋定则确定电流方向。如果线圈端为北极，则从那一端看去电流方向为逆时针。

6. The AC Generator (Alternator) | 交流发电机（交流发电机）

An AC generator consists of a coil of wire rotating between two permanent magnets or an electromagnet. The ends of the coil are connected to two separate slip rings, each of which rubs against a fixed carbon brush. This arrangement allows the coil to rotate while maintaining electrical contact without tangling the wires.

交流发电机由一个在永磁体或电磁体两极间旋转的线圈构成。线圈的两端分别连接到两个独立的滑环上，每个滑环与固定的碳刷接触。这种结构使线圈能够旋转，同时保持电接触而不会缠绕导线。

As the coil rotates, each side cuts through the magnetic field, and an EMF is induced. The induced voltage varies sinusoidally, producing an alternating current (AC) because the slip rings remain connected to the same sides of the coil. Every half turn, the connections are reversed, so the current changes direction twice per cycle. The graph of EMF against time is a sine curve.

线圈旋转时，每一边都切割磁场，产生感应电动势。感应电压按正弦变化，因为滑环始终与线圈的同一侧连接，每转半圈连接方向不变？等等，需要纠正：交流发电机使用滑环，每个环一直连接线圈同一边，因此外部电路连接方式不变，但随着线圈在磁场中的位置变化，感应电压方向每半圈改变一次，从而输出交流电。正确：滑环保持同一边的连接，因此当线圈旋转时，外部电路通过碳刷从滑环得到的电流方向随线圈旋转而自然交替。实际上，交流发电机的输出电流每半圈换向一次。图形是正弦波。所以描述：Every half turn, the induced voltage reverses direction because the side of the coil that was moving up now moves down. The graph of induced EMF against time is a sine wave.

线圈每转半圈，感应电压的方向反转一次，因为原来向上运动的线圈边现在向下运动。感应电动势随时间变化的图形是正弦波。

7. The DC Generator (Dynamo) and Split-Ring Commutator | 直流发电机与换向器

A DC generator is very similar to an AC generator, but instead of two slip rings, it uses a single split-ring commutator. The split ring is a conducting ring cut into two halves, each attached to one end of the coil. As the coil rotates, the two halves of the commutator swap contact with the brushes every half turn.

直流发电机与交流发电机非常相似，但它不使用两个滑环，而是使用一个单一的换向器（split-ring commutator）。换向器是一个切成两半的导电环，每半分别连接线圈的一端。随着线圈旋转，换向器的两个半环每转半圈就交替与碳刷接触。

This arrangement ensures that the current in the external circuit always flows in the same direction, even though the induced voltage in the coil reverses every half turn. The output is a unidirectional but varying DC, often shown as a rectified sine wave (bumpy DC). The voltage never drops below zero but goes from zero to a peak and back to zero twice per cycle.

这种结构确保外部电路中的电流始终沿同一方向流动，即使线圈中的感应电压每半圈改变一次方向。输出的是单向但变化的直流电，通常显示为整流正弦波（脉动直流）。电压从不低于零，而是每周期两次从零到峰值再回到零。

You should be able to compare AC and DC generator graphs and explain the roles of slip rings and commutators.

你应该能够比较交流和直流发电机的波形图，并解释滑环和换向器的作用。

8. Transformers – Structure and Principle | 变压器的结构与原理

A transformer consists of two insulated coils of wire wound around a soft iron core. The coil connected to the input voltage is called the primary coil, and the coil delivering the output voltage is the secondary coil.

变压器由两个缠绕在软铁芯上的绝缘线圈组成。连接输入电压的线圈称为初级线圈，输出端线圈称为次级线圈。

When an alternating current flows in the primary coil, it produces a constantly changing magnetic field in the iron core. This changing magnetic field passes through the secondary coil and induces an alternating voltage across it by electromagnetic induction. Because the iron core channels the magnetic field, almost all the flux links both coils, making the process very efficient.

当初级线圈中有交流电通过时，会在铁芯中产生持续变化的磁场。这个变化的磁场穿过次级线圈，通过电磁感应在次级线圈两端产生交流电压。由于铁芯导引磁场，几乎所有的磁通量都链接两个线圈，因此效率非常高。

Transformers only work with AC, not DC, because a steady DC current would produce a constant magnetic field that does not change, so no voltage would be induced in the secondary coil.

变压器只能使用交流电而不能使用直流电，因为稳定的直流电会产生恒定的磁场，不会变化，因此次级线圈中不会感应出电压。

9. The Transformer Equation and Calculations | 变压器方程与计算

For an ideal transformer (100% efficient), the ratio of the voltages across the primary (Vₚ) and secondary (Vₛ) coils is equal to the ratio of the number of turns on the primary (Nₚ) and secondary (Nₛ) coils. This is expressed as:

对于理想变压器（100% 效率），初级线圈电压 (Vₚ) 与次级线圈电压 (Vₛ) 之比等于初级线圈匝数 (Nₚ) 与次级线圈匝数 (Nₛ) 之比。公式表示为：

Vₚ / Vₛ = Nₚ / Nₛ

This equation can be rearranged depending on which variable you are asked to find. If Nₛ is greater than Nₚ, the transformer is a step-up transformer, increasing voltage. If Nₛ is less than Nₚ, it is a step-down transformer, decreasing voltage.

根据要求解的变量，该方程可以重新排列。如果 Nₛ 大于 Nₚ，则为升压变压器，升高电压；如果 Nₛ 小于 Nₚ，则为降压变压器，降低电压。

Calculations often involve substituting three values into the equation and solving for the fourth. Remember to show your working clearly and state whether the transformer steps up or steps down. In an ideal transformer, input power equals output power, so Vₚ Iₚ = Vₛ Iₛ, which means if voltage is stepped up, current is stepped down, and vice versa.

计算通常要求将三个值代入方程并求解第四个值。记得清楚展示解题步骤，并说明变压器是升压还是降压。在理想变压器中，输入功率等于输出功率，即 Vₚ Iₚ = Vₛ Iₛ，这意味着电压升高时电流降低，反之亦然。

10. The National Grid and Power Transmission | 国家电网与电力传输

Electricity is transmitted from power stations to homes and factories via the National Grid. To reduce energy losses in the cables, the voltage is stepped up to a high value (e.g., 400,000 V) using step-up transformers before transmission. A high voltage means a low current for the same power, so resistive heating (I²R) losses in the cables are minimised.

电力从发电站通过国家电网输送到家庭和工厂。为了减少电缆中的能量损耗，在传输前使用升压变压器将电压升高到很高的值（例如 400,000 伏）。高电压意味着相同功率下电流较小，因此电缆中的电阻热损耗 (I²R) 最小化。

Near the point of use, step-down transformers reduce the voltage to safer levels (e.g., 230 V for homes). The transformer equation is fundamental to understanding why the Grid operates at high voltage and how the turns ratio is selected for each transformer substation.

在用电地点附近，降压变压器将电压降低到安全水平（例如家庭用电 230 伏）。变压器方程是理解电网为何采用高压运行以及每个变电站如何选择匝数比的基础。

The exam may ask you to explain why high voltage transmission reduces power loss or to calculate the turns ratio needed for a given voltage change. Using the relationship between power, voltage, and current (P = IV) alongside the transformer equation is key.

考试可能会要求你解释为什么高压输电能减少功率损耗，或者计算特定电压变化所需的匝数比。将功率、电压和电流的关系 (P = IV) 与变压器方程结合使用是关键。

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Electromagnetic Induction for IGCSE WJEC Physics | IGCSE WJEC 物理：电磁感应考点精讲